Signal integrity has become a major concern in communication systems using high data rates such as 25 Gbps or higher.
Signal degradation can be caused by transmitting a signal through a bandwidth-limited channel, where the transmission frequency is significantly greater than a cutoff frequency. Passing a signal through such a channel results in attenuation of components at higher frequencies, which affects the shape of a pulse arriving at a receiver. Not only is the shape of the pulse within its symbol period changed, but the pulse is also spread out over the subsequent symbol periods. Consequently, when a signal is transmitted through such a channel, the spread pulse of each individual symbol will interfere with following symbols.
The bandwidth limitation can be due to the physical properties of the active components or a medium. For example, at bit rates of 25 Gbps, a copper cable may have a cutoff frequency above which practically none of the transmitted signals will propagate. Therefore, in order to provide sufficient bandwidth for high speed data transmission applications of bit rates of 25 Gbps and above, optical links are often used. Transmission of data over an optical link however, involves conversion of electrical digital signals into optical signals.
A laser diode, such as a Vertical Cavity Surface Emitting Laser (VCSEL), is conventionally used for converting electrical signals into optical signals. However, the laser diode is a non-linear device, and therefore the laser diode affects the shape of its output pulse. Problematically, this pulse distortion can cause intersymbol interference.
FIG. 1 shows a schematic of a conventional transmitter for converting an electrical digital signal into an optical signal and outputting the optical signal to an optical fiber. The conventional transmitter 100 comprises a pre-emphasis driver 101 (that pre-distorts the signal) and a laser diode 102 connected to the pre-emphasis driver 101. The pre-emphasis driver 101 receives at its input an electrical signal, pre-emphasizes the received signal, and outputs pre-distorted electrical pulses. The laser diode 102 receives at its input such electrical pulses from the pre-emphasis driver 101, converts the received electrical pulses into optical signals, and outputs the optical signals into the optical fiber (not shown in FIG. 1).
The pulse shaping performed by the pre-emphasis driver 101 is shown in FIG. 2. The curve 201 represents the shape of a pulse of a digital (binary) signal input to the pre-emphasis driver 101. This pulse makes a first, positive transition from a low level (“0”-level) to an high level (“1”-level), remains approximately constant at the high level during the pulse length, and thereafter makes a second, negative transition from the high level to the low level. For clarity, the “1”-level is defined as the high level, the “0”-level is defined as the low level, a positive transition is defined as a transition from the low level to the high level, and a negative transition is defined as a transition from the high level to the low level of the signal. The curve 202 represents the shape of the signal output by the pre-emphasis circuit 101 when the square pulse 201 is applied to its input. This curve exhibits an overshoot immediately after the positive transition from “0”-level to “1”-level and an undershoot immediately after the negative transition from “1”-level to “0”-level. Particularly, the curve 202 makes a transition from “0”-level to “1”-level, overshoots the “1”-level, relaxes to the “1”-level, makes a transition from “1”-level to “0”-level, undershoots the “0”-level, and then relaxes to the “0”-level.
Hence, the pre-emphasis driver 101 of the conventional transmitter 100 emphasizes/peaks an electrical digital signal during and/or immediately after the transition from one binary signal level to the other binary signal level.
FIG. 2 also shows the shape of the signal output by a bandwidth limited component being excited with a signal without such a pre-distortion, and the shape of the signal output by the bandwidth limited component being excited with a signal having such a pre-distortion. Curve 212 exhibits more steep transitions between the low and high level compared to the curve 211. For the case of a non-linear device showing overshoot and ringing, when the pulse 201 and 202 are applied at its input, one will observe that the shape of the output signal looks like curve 221 and 222, respectively. Here, the pulse 222 exhibits shorter transition times (faster edges) compared to the pulse 221. In other words, pre-emphasis/peaking of the electrical pulse during and/or immediately after the transition from one binary signal level to the other binary signal level shortens the transition times at the edges of the pulse. The description of the behavior of the nonlinear component is detailed in FIG. 3.
FIG. 3 shows optical signals (in terms of optical power) output by the laser diode 102 when driven by the pre-emphasis driver 101. The curve 302 represents the shape of an optical pulse output by the laser diode 102, when an electrical pulse corresponding to curve 202 in FIG. 2 showing pre-emphasis during/after the transition from one binary level to the other, applied at the input of the laser diode 102. The curve 301 represents the shape of an optical pulse output by the laser diode 102, when an electrical pulse corresponding to curve 201 of FIG. 2 showing an electrical pulse without pre-emphasis, is applied at the input of the laser diode 102.
The curve 301 exhibits an overshoot after the transition from “0”-level to “1”-level and an undershoot after the transition from “0”-level to “1”-level. This overshoot/undershoot is due to the self-resonance (relaxation oscillation) of the laser diode 102, and therefore is intrinsic to the optical pulse output by the laser diode 102. Particularly, the curve 301 makes a transition from “0”-level to “1”-level, overshoots the “1”-level by the intrinsic overshoot, settles to the “1”-level, makes a transition from the “1”-level to the “0”-level, undershoots the “0”-level by the intrinsic undershoot, and then settles to the “0”-level.
Curve 302 of FIG. 3 also exhibits an overshoot after the transition from “0”-level to “1”-level and an undershoot after the transition from “1”-level to “0”-level. The overshoot of curve 302 exceeds the intrinsic overshoot of the laser diode 102 by the overshoot that is induced by the pre-emphasized electrical pulse applied at the input of the laser diode 102. The undershoot of curve 302 exceeds the intrinsic undershoot of the laser diode 102 by the undershoot that is induced by the pre-emphasized electrical pulse applied at the input of the laser diode 102. Particularly, the curve 302 makes a transition from “0”-level to “1”-level, overshoots the “1”-level by a resultant overshoot that is induced by both the intrinsic overshoot of the laser diode 102 and the overshoot of the pre-emphasized electrical input pulse, then relaxes to the “1”-level, makes a transition from “1”-level to “0”-level, undershoots the “0”-level by a resultant undershoot that is induced by both the intrinsic undershoot of the laser diode 102 and the undershoot of the pre-emphasized electrical input pulse, and then relaxes to “0”-level.
The effect of the pre-emphasis driver 101 on the optical signal output by the laser diode 102 becomes evident when comparing the curves 302 and 301 of FIG. 3 to each other. This comparison shows that the transition times from one optical power level to another optical power level are shorter for curve 302. In other words, pre-emphasizing/peaking the input pulse of the laser diode 102 during and/or immediately after the transition from one binary level to the other binary level steepens the edges of the optical signal output by the laser diode.
However, the large overshoot of the optical pulse enhances the possibility of oscillations during the relaxation to the “1”-level. Also, the large undershoot of the optical pulse enhances the possibility of oscillation during the relaxation to the “0”-level. These relaxation oscillations prolong the time required for the stabilization of the optical signal around an optical power level. If, however, the pulse duration is too short to ensure stabilization (setting) of the optical signal, intersymbol interference may occur. This is detrimental to signal integrity of the optical signal.